In order to cope with recent economic statuses and rapidly increasing data quantity, the demands of users are soaring to suppress the investment costs of IT devices and optimize the cost-effectiveness. In order to respond to such demands, a capacity virtualizing function in storage systems called thin provisioning or dynamic provisioning (hereinafter referred to as DP function) is provided.
The DP function allocates a storage area (pool page) of given storage units to the virtual volume that the host computer uses when a write request is output from a host computer to a storage system.
Recently, a dynamic relocation function called dynamic tiering (hereinafter referred to as DT function) taught in patent literature 1 having further advanced the capacity virtualizing function has been proposed.
According to the DT function, a pool is composed by combining a plurality of storage drives having different cost performances and different bit costs (SSD (Solid State Drive), SAS (Serial Attached SCSI), SATA (Serial ATA) etc.), and virtual volumes are composed from the pools having been divided into tiers.
Then, the access frequency (I/O count) is measured per page unit as described, wherein the pages having high access frequencies are migrated to high speed upper tier drives such as SSDs and pages having low access frequencies are migrated to slow lower tier drives such as SATAs, in order to automatically optimize the load caused by the various access frequencies of respective drives.